This invention relates to a method and apparatus for biologically and mechanically drying organic material such as garbage, waste, sewage, pulp sludge, food, manure, renderings, wood, agricultural waste, bagasse, peat, mash, silage, biomass, feedstock, etc., particularly for the preparation of a useful fuel but also for products such as DDGS (Distillers Dried Grains with Solubles) or other valuable Nutraceuticals or feed.
A number of attempts have been made to dry wet organic materials and wet biomass into a sufficiently dry state to be useful as combustible solid fuel. A number of these involve the direct application of externally generated heat to the raw material, for example by the use of rotary and other dryers. In the method and apparatus for preparing organic waste fuels which is the subject of U.S. Pat. No. 4,254,716 (Graham) a dryer, either of a single or multiple pass, or fluid bed design makes use of hot flue gases from an energy recovery boiler.
However, known methods for drying wet organic materials are impractical for preparing useful fuels by reason of their inherent inability to reduce the water content of the organic waste to a sufficiently low level in an energetically economic way. The water content of the dried organic waste must be brought down to about 20%, and to do so conventional processes consume undue amounts of energy (fossil fuel, electricity). Consequently, the end product is not commercially useful as a fuel in competition with substances such as coal, etc.
Methods of making combustible briquettes or pellets from waste products using mechanical means (belt presses, screw presses, rotary presses and centrifuges) have been known in the art. U.S. Pat. No. 3,910,775 (Jackman) describes a method and apparatus for processing waste material in which the refuse is dewatered in a rolling mill and formed into briquettes. U.S. Pat. No. 4,496,365 (Lindemann) describes a method of producing briquettes from organic waste products, but requires the enrichment of these waste products with organic materials of high heating value, such as coal, before compressing the mixture under high pressures and exothermal heating due to chemical reaction of lime (CaO) to produce a product having the desired high heating value. Mechanical means as such can be effective to remove free (interstitial) water from organic materials, but are largely incapable of removing water from within the constituent cells of the raw organic waste, and so are incapable of reaching the lower levels of water content necessary to create a practical fuel.
It is a principal object of my invention to provide a method for the highly efficient drying of organic material and biomass to a state in which use of the end product as a solid fuel is commercially practical and economical, and to devise apparatus for carrying out the method.